What's the difference between a field image and a dark one?

Brightfield and Dark field illumination

Understanding "W":

The reflected light is the same angle as the light source. Brightfield: The light bounces back into the camera. Dark-Dark: The camera is not entered after the light is reflected.

Ming Field Lighting:

• Good for forming high contrast, but reflective surfaces produce specular reflections

• The light source range should be twice times the field of view of the camera lens

• Avoid "hot spots": Diffuse light sources provide uniform illumination at Brightfield.

Dark-Dark Lighting:

• Diffuse light is reflected into the camera but specular light is reflected away

• The light source is outside the "W"

• The light is reflected without entering the camera except for the textured surface and the convex height-changing surface

What's the difference between a brightfield and a dark-field image?

One is transmission beam imaging, one is diffraction beam imaging.

TEM images are divided into microscopic and diffraction patterns of samples, the two are different electron imaging, the former is transmission electron imaging, the latter is scattering electron imaging.
In transmission electron microscope, not only the specific image area can be selected for electron diffraction (selective electron diffraction), but also the imaging electron beam can be selected. (Select diffraction imaging)
Brightfield (BF): Use direct electrons to form the image (transmission beam), like clear.
Dark-Dark Image (DF): The image (diffraction beam) formed by scattering electrons, such as distortion and low resolution.
The choice of imaging electrons is achieved by inserting an objective aperture on the back focal surface of the objective lens.
Center Dark-Dark Image (CDF): The incident electron beam is inclined to illuminate the specimen, resulting in a dark-dark image. Like no distortion, high resolution.
Under the condition of dark-dark imaging, the imaging electron beam deviates from the optical axis of transmission electron microscope, resulting in a large aberration, poor imaging quality, and a central dark-dark image for obtaining high quality dark image. That is, the incident electron beam is tilted backward to a corresponding scattering angle, which propagates the scattering electrons along the optical axis.

The so-called Brightfield, dark-field imaging is only when the low-magnification observation said.
Generally speaking, we all prefer to use Brightfield, because the imaging lining is good (especially with the appropriate diaphragm), and the deformation is small. Its main performance is the thickness of the lining, sensitive to the thickness.

When observing defects such as dislocation, the twins like to use a dark-dark image, because the dark area is like a selected diffraction beam, which corresponds to the crystal-specific crystal surface. In the defective place, the direction of the electron diffraction is different from the complete place, thus allowing the defective place to be clearly displayed on the Dark field image. The Ming-field image is not sensitive to defects because it is a plurality of diffraction beams, although sometimes it can reflect defects, but it is fuzzy. Its main performance is diffraction contrast. Which is sensitive to the diffraction surface.

For example, a twin material, for Brightfield, the twin interface is very light, but choose the appropriate diffraction point to do dark-field image can be very clearly see the twin interface. and choose one of the crystal specific diffraction points, do dark dark can be found only this one crystal appears on the image, and another crystal is not visible.

An important use for the dark field is to observe layer faults, such as the 111-direction layer fault in the cubic crystal system, which can not be seen with Brightfield. Because of the same thickness as defects and defect-free places. However, dark-field looks like a triangular or butterfly-shaped or even pyramid-lined shade when viewed in a particular direction. A TEM technique used at this time is called a double beam.

What's the difference between a field image and a dark one?